git cherry-pick says "...38c74d is a merge but no -m option was given"

The way a cherry-pick works is by taking the diff a changeset represents (the difference between the working tree at that point and the working tree of its parent), and applying it to your current branch.

So, if a commit has two or more parents, it also represents two or more diffs - which one should be applied?

You're trying to cherry pick fd9f578, which was a merge with two parents. So you need to tell the cherry-pick command which one against which the diff should be calculated, by using the -m option. For example, git cherry-pick -m 1 fd9f578 to use parent 1 as the base.

I can't say for sure for your particular situation, but using git merge instead of git cherry-pick is generally advisable. When you cherry-pick a merge commit, it collapses all the changes made in the parent you didn't specify to -m into that one commit. You lose all their history, and glom together all their diffs. Your call.

-m means the parent number.

From the git doc:

Usually you cannot cherry-pick a merge because you do not know which side of the merge should be considered the mainline. This option specifies the parent number (starting from 1) of the mainline and allows cherry-pick to replay the change relative to the specified parent.

For example, if your commit tree is like below:

- A - D - E - F -   master
   \     /
    B - C           branch one

then git cherry-pick E will produce the issue you faced.

git cherry-pick E -m 1 means using D-E, while git cherry-pick E -m 2 means using B-C-E.

Simplify. Cherry-pick the commits. Don't cherry-pick the merge.

If you determine you need to include the merge vs cherry-picking the related commits, you have two options:

1. (More complicated and obscure; also discards history) you can indicate which parent should apply.

• Use the -m option to do so. For example, git cherry-pick -m 1 fd9f578 will use the first parent listed in the merge as the base.

• Also consider that when you cherry-pick a merge commit, it collapses all the changes made in the parent you didn't specify to -m into that one commit. You lose all their history, and glom together all their diffs. Your call.

2. (Simpler and more familiar; preserves history) you can use git merge instead of git cherry-pick.

• As is usual with git merge, it will attempt to apply all commits that exist on the branch you are merging, and list them individually in your git log.

@Borealid's answer is correct, but suppose that you don't care about preserving the exact merging history of a branch and just want to cherry-pick a linearized version of it. Here's an easy and safe way to do that:

Starting state: you are on branch X, and you want to cherry-pick the commits Y..Z.

1. git checkout -b tempZ Z
2. git rebase Y
3. git checkout -b newX X
4. git cherry-pick Y..tempZ
5. (optional) git branch -D tempZ

What this does is to create a branch tempZ based on Z, but with the history from Y onward linearized, and then cherry-pick that onto a copy of X called newX. (It's safer to do this on a new branch rather than to mutate X.) Of course there might be conflicts in step 4, which you'll have to resolve in the usual way (cherry-pick works very much like rebase in that respect). Finally it deletes the temporary tempZ branch.

If step 2 gives the message "Current branch tempZ is up to date", then Y..Z was already linear, so just ignore that message and proceed with steps 3 onward.

Then review newX and see whether that did what you wanted.

(Note: this is not the same as a simple git rebase X when on branch Z, because it doesn't depend in any way on the relationship between X and Y; there may be commits between the common ancestor and Y that you didn't want.)